The present invention relates to noise attenuation panels and is particularly, although not exclusively, concerned with noise attenuation panels for use in the attenuation of noise in aero engines.
As is schematically illustrated in FIG. 1 a typical aero engine 25 includes a turbofan power unit 26 mounted within a nacelle 27 suspended from a pylon 32. The nacelle 27 includes a nose cowl 28 having an outer wall 29 and an inner wall 30. The inner wall 30 is in part formed by noise attenuation panels P. The panels P are arranged to form part of the inner wall of the nose cowl 28 in such disposition that the outer facing sheet of the panel forms the wall surface defining the air intake duct 31 for the power unit 26, The panels P in this disposition serve to reduce noise created by the high speed flow of air passing though the duct 31 and into the power unit 26, as well as to reduce noise generated by the fan blades of the unit 26.
As shown in FIG. 2, a typical noise attenuation panel 10 comprises a backing sheet 11, a honeycomb core 12 and a facing component part 13 comprising outer and inner facing sheets 131 and 132.
The core 12 comprises a multiplicity of open ended juxtaposed cells 15 of hexagonal cross section. The walls of the cells 15 extend from the front face of the core 12 to the rear face. Each cell 15 is, however, divided into an upper subcell 151 and lower subcell 152 by a septum element 14.
The outer facing sheet 131 of the facing component part 13 takes the form of a woven stainless steel mesh. The inner sheet 132 of the facing component part 13 is an open weave fabric formed from a carbon fibre/resin matrix composite material, the weave being such as to provide apertures constituted by the openings between adjacent warp and weft threads of the fabric. The fabric is preferably so woven as to produce a proportion of open aperture area relative to the total surface area of the sheet of say 30%. The fabric is also so woven that a relatively large number of its apertures are contained within the bounds of each cell 15 of the honeycomb core 12.
The outer facing sheet 131 is bonded to the inner facing sheet 132 and the inner facing sheet 132 is secured to the upper face of the honeycomb core 12 by means of an epoxy resin adhesive.
The backing sheet 11 is unperforated and made from a non-porous impermeable sheet material and is secured by an epoxy resin adhesive to a lower face of the honeycomb core 12.
The walls of the cells 15 of the core 12 are made from a non-porous impermeable sheet. The cells 15 are preferably provided with drainage slots 16 to allow for condensates to drain from the panel 10.
The panel 10 is typically of arcuate form, possibly of double curvature, and is embodied as a structural part of a duct of a nose cowl of the turbofan aero engine, the panel 10 being one of several arcuate panels P disposed just upstream of the fan of the engine.
Such noise attenuation panels when used in aeroengine nacelles are termed acoustic liners and absorb engine intake noise by allowing a controlled resonance to occur with partially closed honeycomb cells.
In a typical manufacturing procedure for such panels, the following steps are carried out:
(1) The backing sheet 11 is precured
(2) The open weave inner facing sheet 132 is precured to a predetermined profile
(3) The cured inner facing sheet 132 is bonded to the stainless steel mesh outer facing sheet 131
(4) Adhesive is reticulated onto the walls of the cells of the honeycomb core 12.
(5) The above components are assembled and bonded together, that is to say, the backing sheet 11, the honeycomb core 12 and the pre-bonded outer and inner sheets 131, 132.
The application of adhesive to the honeycomb core 12 is typically carried out as illustrated in FIG. 3(A) to FIG. 3(F) and comprises the following steps:
A) Adhesive film 101 is applied to the face of honeycomb core 12, as illustrated in FIG. 3(A),
B) The adhesive film 101 is heated so that it tacks to side walls of the cells 15 of the honeycomb core, as illustrated in FIG. 3(B),
C) Hot air is applied to the adhesive film 101 in the direction of the arrows R to cause the adhesive film 101 to balloon while thinning the film at the mid point of each cell, as illustrated in FIG. 3(C),
D) The adhesive film 101 is caused to burst and starts to reticulate, as illustrated in FIG. 3(D),
E) The adhesive film fully reticulates to envelop the ends of the cell walls and form beads 102, as illustrated in FIG. 3(E), and
F) Special heat treatment is applied to improve containment of the reticulated adhesive, as illustrated in FIG. 3(F).
During the final stage bonding of the pre-bonded sheets 131 and 132 to the honeycomb core 12 adhesive bleeds through the outer facing sheet 132 causing cosmetic spots 103 as illustrated in FIG. 4. Furthermore, the volume of adhesive deposited around the honeycomb cell edges is non-uniform.
In an attempt to control adhesive flow, an oven stabilisation cycle was introduced after reticulation and prior to final stage cure. It causes adhesive to flow away from the cell edge and also introduces a degree of cure advance. In theory a more uniform adhesive bead 102 is formed with a higher initial viscosity which is less prone to excessive flow during final stage cure.
It has however been found that oven stabilisation is not always successful and that a critical size adhesive droplet 103 will flow through a typical open area intersection as shown in FIG. 4.
The spotting effect is unpredictable and a function of reticulation, stabilisation and autoclave cure combined with adhesive chemistry/viscosity.
It is an object of the present invention to provide a noise attenuation panel so constructed as not to give rise to the above-mentioned adverse spotting effects at the front surface of the outer facing sheet of a noise attenuation panel.
According to a first aspect of the present invention there is provided a method of manufacturing a noise attenuation panel which comprises:
a cellular component part which has a front face, a rear face and wall portions which extend from the front face to the rear face and which provide bounding surfaces for a multiplicity of cells which extend from the front face to the rear face, and
a facing component part which:
has a front face and a rear face,
extends across the ends of the cells of the cellular component part at the front face therof with the rear face of the facing component part adjacent the front face of the cellular component part,
is formed with a multiplicity of apertures which provide gaseous fluid communication between the cells of the cellular component part and the front face of the facing component part for the attenuation of noise generated by gaseous fluid flow over the surface of the front face of the facing component part, the method comprising:
bonding the facing component part to the cellular component part by the steps of:
applying an adhesive film to the front face of the cellular component part
causing the film to reticulate to the ends of the walls of the cells at the front face of the cellular component part,
introducing an adhesive flow control sheet between the front face of the cellular component part and the rear face of the facing component part,
bringing the two component parts together, with the interposition of the adhesive flow control sheet, and
causing the reticulated adhesive on the ends of the wall portions of the cells of the cellular component part to bond the two component parts together with adhesive flow to the facing component part under the control of the adhesive flow control sheet.
In an embodiment of the invention according to its first aspect, the rear face of the outer facing sheet and the front face of the inner facing sheet are bonded to form the facing component part prior to the step of bringing the facing and cellular component parts together for bonding together by the reticulated adhesive.
In an embodiment of the invention according to its first aspect the adhesive flow control sheet is a low areal/weight fibre dominated control sheet. The control sheet may be formed from or includes fibres which are randomly chopped and distributed, knitted or woven.
In an embodiment of the invention the control sheet is in the form of an open weave fabric such as a scrim and the fibres of the fabric are glass, polyester, aramid or carbon fibres.
In accordance with an embodiment of the invention hereinafter to be described, the facing component part comprises an outer facing sheet having a front face and a rear face and formed with a multiplicity of apertures which provide gaseous fluid communication between the front face and the rear face thereof, and an inner facing sheet having a front face and a rear face and formed with a multiplicity of apertures which provide gaseous fluid communication between the front face and the rear face. The rear face of the outer facing sheet is bonded to the front face of the inner facing sheet, and the reticulated adhesive during bonding of the two component parts flows under the control of the adhesive flow control sheet to bond the rear face of the inner facing sheet to the ends of the walls of the cellular component part. Reticulated adhesive flow is then so controlled by the adhesive flow control sheet as to penetrate and bond to the inner facing sheet without penetration to the outer facing sheet.
The outer facing sheet may take the form of a mesh forming a regular array of apertures opening on to the front face thereof. The mesh may be a woven stainless steel mesh. The inner facing sheet of the facing component part may be in the form of an open weave fabric such as a Hexcell open weave fabric.
According to a second aspect of the invention there is provided a noise attenuation panel produced by the method according to the first aspect of the invention.